Alkylation of phenols with olefins in the presence of acid catalysts is a known process. It is little used in industry however because of low reaction yields. In U.S. Pat. No. 2,832,808 and 2,722,556, for instance, alkylation of phenols is described with olefins in the presence of acid catalysts such as sulphuric acid or phosphoric acid. However, according to these processes, one obtains very low yields, even below 50%, in monoalkylated products or one obtains selectivity to dialkylated products over 90%.
In Japanese Patent Applications 58/140.035 and 59/112.935 the above mentioned drawback is partly overcome. In these applications the preparation of monoalkyl substituted hydroquinone is described by reaction with vinyl aromatic hydrocarbons to obtain reaction yields of the order of 90-93%. However, these processes also are not free from drawbacks, as the alkylation, to have satisfactory results, has to be carried out either by using the reaction product as solvent or by letting the reaction take place in the presence of a polysubstituted hydroquinone.
In U.S. Pat. No. 4,661,645 the synthesis is described of (1-phenylethyl)hydroquinone starting from styrene and hydroquinone in the presence of a Lewis acid and in a homogeneous phase, by using an alkyl ether as homogenisation solvent. In this case the reaction product also contains high quantities of di(phenylethyl)hydroquinone, computable in values over 30% by weight with respect to the mixture of mono and di-substituted products.
The applicant showed in the Italian Patent Application No. 22821 A/87 (filed on Dec. 1, 1987) that drawbacks of the prior art could be overcome by a process for alkylation of phenols, in which the reaction was carried out in a heterogeneous phase. In said application a process is described for alkylation of phenols comprising letting a phenol react with a vinyl-aromatic hydrocarbon in the presence of an organic solvent and of a catalyst which is an inorganic acid diluted in water. According to the process of the above mentioned application one can obtain a yield in monoalkylated product over 95% and a selectivity to dialkylated products below 5%, with a purity relating to mono and di-alkylated products over 99.9%. However, if use is made for the alkylation of a vinyl-aromatic hydrocarbon substituted in the vinyl group with an alkyl radical, such as for instance alpha-methyl-styrene, the alkylation yields, calculated on olefin, are low because those hydrocarbons are very reactive and tend to give oligomers like the following: ##STR1##